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Here is a plot of the effect of exit point altitude on the altitude loss that occurs on a perfect wingsuit BASE jump during the subterminal flight.

If you trace your terminal glide path back to the cliff, then the subterminal altitude loss is defined as the height from this point to the exit point. The results are produced from a theoretical model, which is in turn based on accurate GPS data of me flying my V3, but general trends may be of interest to all wingsuit BASE jumpers, and I'd be interested to know if the results agree with their own experiences. For example, note that about 50 additional metres are required to get flying when jumping the Mushroom (about 3250 m above sea level) as compared to the Valley (about 1450 m).

The results already account for typical changes in temperature with altitude. The effects of temperature variation are in any case very small, at about 0.66 m/K. The effects of air pressure changes during normal weather conditions are also very small; perhaps +-3m. The effects of a poor exit are enormous by comparison! Even an exit that feels OK, might cost me an additional 20 m. I'm working on reducing this.

Geo.

Nice work.

Losing an additional 50 meters at 3250m above sea level seems fairly significant. Is this the consensus? What about exiting above 4000m msl, like the heiger?

Nice work, Geo, keep it up!

Things like these are very educational (and it's better to educate yourself about "surprises" like this one on the ground wishing you were in the air than the other way around ). The eye picture from the exit could be very similar to what you've seen at lower altitudes ("I can easily outfly that ledge at 5s rockdrop!"), but the eye fails to feel how thinner the air is and how less lift you'll develop in the first few seconds into flight (in the first few seconds while your speed is mostly dictated by gravity, not air, the aerodynamic forces will be lower in the direct proportion to lower air density, so if the air is 1.5x thinner, you'll move away from the wall 1.5x slower initially).

Wingsuit Studio comes up with similar results:

1. A steep, high L/D dive at normalized sustained speeds (sea level) of Vxs = 120km/h, Vys = 48km/h, L/D = 2.5 results in:

- at exit altitude 1300m (typical for Lauterbrunnen Valley exits):

164m start-to-fly height, 4.2s start-to-fly time *

- at exit altitude 3700m (Ubermushroom exit):

206m start-to-fly height, 4.7s start-to-fly time

Difference: ~40m more altitude lost due to thinner air.


2. A more floaty, medium L/D, "clear-short-rockdrop" launch at normalized sustained speeds (sea level) of Vxs = 90km/h, Vys = 60km/h, L/D = 1.5 results in:

- at exit altitude 1300m (typical for Lauterbrunnen Valley exits):

122m start-to-fly height, 4.2s start-to-fly time

- at exit altitude 3700m (Ubermushroom exit):

153m start-to-fly height, 4.7s start-to-fly time

Difference: ~30m more altitude lost due to thinner air.


I guess, for giant, mattressy, slow flying suits the absolute difference will be less, although relative loss of altitude will be about the same.

Attached animated GIF illustrates the simulations.

Yuri


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* <-- GreenMachine's heart must be pumping seeing this, hahaha

Start-to-fly height is defined as height which when subtracted from altitude used gives distance/height ratio equal to calculated L/D. In other words, start-to-fly height gives correct L/D using "old" method of calculating L/D from a BASE jump.

Start-to-fly time is defined as time which when subtracted from time of your flight gives time it would take to fly from start-to-fly height point to the final point of your flight at sustained speed calculated by L/D Calculator (and properly adjusted for altitude and temperature).

To put it differently, if your clone were flying at sustained speed of your flight mode as to reach the same target point, he will cross the face of the mountain at start-to-fly height below the exit and reach the target start-to-fly time before you.

I've been asked some questions about this plot by PM, which I'll answer here. I use the MTi-G from Xsens, a MEMS and GPS based motion capture device. It's designed for scientific applications so it's pretty expensive, but it provides just about all the flight data you could possibly wish for - acceleration, velocity, position, roll, pitch, and yaw rates, orientation, and air pressure. I wear it on my chest in order to measure angle of attack. This means I don't have to work in the L/D domain (as I believe Yuri does), and can instead consider only AoA, and have the equations figure out the lift and drag internally.

In order to produce the plot, I used the calculus of variations to solve for the optimal angle of attack as a function of time (this is the non-trivial part), and then calculated the corresponding altitude loss. This optimality is what makes the comparison of different exit altitudes meaningful.

Links:
The MTi-G:
http://www.xsens.com/en/general/mti-g

An article about my research I wrote for the Xsens website
http://www.xsens.com/...ysis-using-the-mti-g

is your setup sensitive enough to note variations in terminal speeds as the altitude (air density) varies?

In theory it is, but in practice I've found that even when trying really hard to maintain a constant speed, fluctuations of at least 10% are inevitable. (Flying at 40 and 45 m/s feels very similar!) The fluctuations are due to small changes in body position, in much the same way that changes to an aircraft's elevator will change its AoA (and thus its speed). Speed will only vary as the inverse of the square root of the air density, and since most of my exits are from similar altitudes, I've not noticed any speed differences. Then again, I haven't been looking for them. The first thing I do when analyzing flight data is normalize them and then work only with non-dimensional parameters.

Geo, can you post the graphs of lift and drag coefficients vs. AoA?

I can't just yet, because I'm waiting to get the results published first. But I will in due course.